The invention generally relates to electronic circuits, and more particularly to methods and apparatuses for near losslessly measuring inductor current.
In switching regulators and particularly multi-phase and other advanced power converters, current through inductors must be accurately measured in order for the control circuitry to timely provide control signals for controlling the operation of the devices. A typical conventional way to measure inductor current is illustrated in FIG. 1.
As shown in FIG. 1, a sampling resistor 10 is connected in series with an inductor 12 and its internal resistance 14. The voltage Vo across sampling resistor 10 is measured by an op-amp 16. From this voltage Vo and the resistance value Rs of sampling resistor 10, the inductor current iL can be derived to be Vo/Rs. This method, however, suffers from significant losses due to the additional power consumed by sampling resistor 10, which in turn causes inefficiency in the circuit performance.
Another conventional way to measure inductor current is illustrated in FIG. 2. As shown in FIG. 2, a RC network 20 composed of a resistor 22 and a capacitor 24 is used to measure the inductor current iL. In this method, the voltage across capacitor 24, Vc, is measured. This voltage Vc is taken as the voltage across the inductor 12, from which the inductor current iL is derived from the expression Vc/Rs. This approach, however, generates significant errors because the load resistance is not taken into account. Practically, this is never the case.
Therefore, there is a need for an improved way of measuring inductor current with minimum losses and errors.
The present invention provides an improved measuring circuit for measuring a current of an inductor with minimum losses and errors. According to one embodiment of the invention, the measuring circuit comprises an op-amp having first and second input terminals and an output terminal, with the second terminal for connecting a first end of the inductor; a RC network connected between the first input terminal and the output terminal of the op-amp; and a scaling resistor having a first end connected to the first input terminal of the op-amp and a second end for connecting to a second end of the inductor. In a specific embodiment, the RC network comprises a resistor and a capacitor connected to each other in parallel. According to the invention, by setting the RC constant of the RC network to be equal to the ratio of the inductor value over its internal resistance value, the inductor current can be derived independent of the frequencies of AC signals.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.